1. Field of the Invention
Currently, the antiferroelectric discovered in chiral liquid crystal has a tristable switching property, i.e., in addition to the bistable property of regular ferroelectric liquid crystal, chiral liquid crystal has an antiferroelectric chiral smectic C phrase property, hereinafter called S.sub.CA *, or the so-called "third stable state".
Normally, antiferroelectric liquid crystal and ferroelectric liquid crystal have a similar helical structure. The helical structure of antiferroelectric liquid crystal is composed of zigzag bilayers in which, as shown in FIG. 1, the liquid crystal molecules of each two adjacent layers are arranged in reversed directions, therefore the helical structure shows a half helical pitch reflection in a selective reflection spectrum (see (c) and (d) in FIG. 4). In the helical structure of ferroelectric liquid crystal, the liquid crystal molecules of each two adjacent layers are arranged in same direction, as shown in FIG. 3, therefore the helical structure shows a full helical pitch reflection in a selective reflection spectrum (see (a) and (b) in FIG. 4).
The helical structure of antiferroelectric liquid crystal can be surface stabilized to the state like ferroelectric liquid crystal by means of the unwound effect between liquid crystal molecules and interface. Under this surface stabilized state, due to that the liquid crystal molecules are arranged in same direction, ferroelectric liquid crystal shows same direction of molecule dipole as shown in FIG. 5. This feature causes regular ferroelectric liquid crystal to produce pure spontaneous polarization. On the contrary, the liquid crystal molecules of antiferroelectric liquid crystal show a zigzag bilayer structure under the surface stabilized state, as shown in FIG. 6, and the dipole of the molecules is respectively set off, without causing pure spontaneous polarization. This state of reverse molecular arrangement is the third stable state, which can be switched to ferroelectric state by means of the application of an electric field. This switching is the so-called "field induced antiferroelectric to ferroelectric switching". Under the effect of an electric field, the tristable state of antiferroelectric shows particular electric-optical effects, for example, the properties of DC critical electric field and hysteresis. These properties can be used to improve design limitations on LCD viewing angle and contrast ratio.
Further, antiferroelectric liquid crystal further advantages as outlined hereinafter:
(1) The optical axes of the molecules of antiferroelectric liquid crystal molecules are arranged along the alignment, which facilitates to alignment stability; PA1 (2) Under the effect of unwound, antiferroelectric liquid crystal has the third stable state, that effectively eliminates ghost effect and memory effect; PA1 (3) Under the effect of an electric field, antiferroelectric liquid crystal has DC critical electric field and retarding properties, therefore it increases matrix addressing capacity and improves LCD resolution; PA1 (4) Antiferroelectric liquid crystal tends to obtain quasi bookshelf alignment structure, therefore it enables the LCD to have high value of contrast ratio (about 20.about.30); PA1 (5) Antiferroelectric liquid crystal has rapid response time, which enables current LCD driving technique to be fully utilized, and therefore it is not necessary to develop a new driving technique; and PA1 (6) Antiferroelectric liquid crystal has a self-alignment recovery property; therefore it greatly improves LCD's mechanical resisting and heat impact resisting capability. PA1 (1) Great tilt angle (&gt;35.degree.); PA1 (2) Low driving voltage (&lt;2V/.mu.m.sup.-1); PA1 (3) Ideal gray scale; PA1 (4) Fast antiferroelectric to ferroelectric switching time (&lt;50.mu.s); PA1 (5) High contrast value (&gt;100); and PA1 (6) Broad viewing angle (&gt;60.degree.).
As indicated above, antiferroelectric liquid crystal plays an important role in the manufacturing and application of photoelectric apparatus. Photoelectric apparatus manufacturers and research units pay much attention to the study of the molecular structure of antiferroelectric liquid crystal materials and the relationship between liquid crystals, so as to design a low-cost, high-performance antiferroelectric liquid crystal material for making LCDs.
2. Description of the Prior Art
The molecular chemical structure of currently developed antiferroelectric liquid crystal materials is similar to ferroelectric liquid crystal molecules. Both are commonly composed of a terminal chiral alkyl chain, a rigid core, a linking group, and a chiral alkyl chain (see FIG. 7). The molecular structure of the terminal chiral alkyl chain, the rigid core and the linking group is the key factor for the formation of antiferroelectric liquid crystal.
The terminal chiral alkyl chain structure in the molecular structure of antiferroelectric liquid crystal has four different kinds as shown in FIG. 8. The polarity of molecular size of the substituent (R.sub.1) of the chiral center C* is the main factor that affects the formation of antiferroelectric liquid crystal. Until now, only in the structure molecules of the third kind shown in FIG. 8 is discovered having no antiferroelectric liquid crystal phase, and the structure change of the rigid core has little effect to the formation of antiferroelectric liquid crystal. In materials having different structures of rigid core, as shown in FIG. 9, the change of the rigid core structure from an aromatic ring to an iso-aromatic ring or the one having a substituent does not affect the formation of antiferroelectric liquid crystal. Further, most rigid core is composed of at least three aromatic rings or iso-aromatic rings. Few antiferroelectric. liquid crystal materials have a two-ring structure.
The linking group in an antiferroelectric liquid crystal molecular structure is normally of ester group or ketone group. As illustrated in FIG. 10, the structure, which is linked between the rigid core and the terminal chiral alkyl chain, is most important. In a recent study on the radiation of X-rays and FTIR spectrum to ester materials, it is reported that chiral tail linking ester group, which is linked by --COO--, may produces a bent structure, causing the molecules at two adjacent layers to form a reverse pair arrangement of zigzag bilayer structure. This --COO-- linking group increases conjugation of the internal electrons of liquid crystal molecules in molecular long axis, that facilitates the formation of antiferroelectric S.sub.CA * liquid crystal.
Because antiferroelectric liquid crystal has tristable switching property, DC critical electric field property (E.sub.th) and retarding property, it is appreciated key material for making high quality LCDs. However, due to the constraint of the influence of high E.sub.th value and pretransitional effect, conventional antiferroelectric liquid crystal materials do not provide broad viewing angle and high contrast ratio as expected, when used in the fabrication of LCDs.
Till 1996, Japanese scientist Inui mixed the three antiferroelectric liquid crystal materials (I, II and III) indicated in FIG. 11 at different ratio, and made a study to see the result of different mixing ratio on E.sub.th value and pretransitional effect. This study shows that changing the mixing ratio of these liquid crystal materials effectively reduces antiferroelectric liquid crystal's E.sub.th value, however the change of the mixing ratio causes pretransitional effect more significant. According to Inui's report, when the mixing ratio of I:II:III=40:40:20, no E.sub.th value is found, and its field-induced antiferroelectric to ferroelectric switching shows a V-shaped switching (see FIG. 12). Inui give the name of "Thresholdless antiferroelectric liquid crystals; TLAFLCs" to this antiferroelectric liquid crystal mixture. These thresholdless antiferroelectric liquid crystals have the following properties:
The aforesaid properties eliminate the gray scale problem occurred during the fabrication of a passive matrix addressing (PM) surface stable ferroelectric liquid crystal display, and also improve the drawback of being difficult to obtain a high contrast ratio commonly existed in regular active matrix (AM) or thin film transistor (TFT) addressing type deformed-helix ferroelectric liquid crystal displays and passive matrix addressing type antiferroelectric liquid crystal displays.